Hello all,

I have a job in which the contractor has vented a 4" PVC bathroom exhaust vent and a 4" PVC bathroom vent within 3 ft. of each other. I can see the underside of both vents through a drop ceiling and I can see where both vents penetrate the flat concrete roof. Both exhaust vent and plumbing vent loop back down once they exit through the roof - but even though they both loop back down with two degree elbows there is still a good 2 feet from the top of the concrete roof to the looped down vent terminations.

My question is, the 4" plumbing vent is continuously dripping from the underside from where it penetrates through the concrete roof - we haven't had rain in weeks, and there is never any water pooling on the concrete roof. I know for a fact that the 4" PVC plumbing vent is not wrapped with any insulation where it penetrates the roof. Could that is what is causing the condensation?

I am a licensed Master Plumber but admittedly have gotten a little rusty over the years.

condensatation forming INSIDE the pipe and dripping from the loop is normal

the PURPOSE of the vent is to VENT warm rising moist air from the plumbing system as well as to admit some air when necessary

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"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638

i would remove the double 90 and extend the 4 inch plumbing vent to about 24 inches over the roof.
you will constantly have a condensing water forming ice inside the pipe but a 4 inch pipe is large enough that it will never block up. any condensation will go back down the vent and if the vent has a slope to it the water will just drain harmlessly away.

I think I will cut off the 4" vent loop and just run the vent straight up. I may have worded my original post wrong, but the issue is actually water - presumedly condensate is dripping onto the ceiling tiles below, dripping from where the 4" PVC vent protrudes through the concrete ceiling (no plumbing fixtures above). Very strange as every time I go up on the concrete roof it is completely dry up there. Somehow, water is forming somewhere between the 6 inches of pipe that travels through the concrete that I can't see. Which got me wondering, could the warm air of the bathroom ceiling and the cold air outside be forming condensate on the exterior of the PVC pipe that goes through the 6" of cement ceiling?

Condensation forms where moist air contacts a cold surface. There should not be enough moisture below the roof to form condensation on the exterior of a pipe. Moist air will be present inside both of your pipes and that moisture will condense on the inner surfrace of the pipe. A leak in a pipe/joiint can allow that condensed moisture to leak outside the pipe and down to the ceiling. I'm assuming the sealing of the pipes through the roof is not a possible issue?

The pipe is cold from outside temp and the air inside house is warm, perfect for humidity inside to condense on pipe, should always wrap a pipe like that.